Press D (which acts as a halant/diacritic) after a consonant. Example: K + D = क् (half ka). Then type a consonant to form a conjunct.
Appendix A — Sample problematic inputs (examples) ism 3.0 keyboard driver marathi
How to Download and Use ISM 3.0 Keyboard Driver for Marathi Typing Press D (which acts as a halant/diacritic) after a consonant